Thermal cracking is widely seen as one of the oldest and well-established processes in conventional refining. The object in conventional refining is to convert a hydrocarbonaceous feedstock into one or more useful products. Depending on feedstock availability and the desired product slate, many hydrocarbon conversion processes have been developed over time. Some processes are non-catalytic such as visbreaking and thermal cracking, others like fluidized catalytic cracking (FCC), hydrocracking and reforming are examples of catalytic processes. The processes referred to herein above have in common that they are geared, and often optimized, to producing transportation fuels such as gasoline and gas oils.
Thermal conversion processes are well known in industry. In particular, the Shell Soaker Visbreaking Process is well known and practiced for many years in many refineries all over the world. For instance, in EPB-7656 a process for the continuous thermal cracking of hydrocarbon oils is described, which has been incorporated herein by way of reference. In this document reference is made to the use of soaker vessels, in particular to soaker vessels containing one or more internals. Preferred configurations comprise up to 20 plates, preferably perforated plates containing round holes having a diameter in the range from 5 to 200 mm. Residence times for the feedstock are suitably in the range from 5 to 60 minutes. Such processes can be carried out upflow or downflow; very good results are normally obtained when operating in upflow mode.
In modern refineries there is a tendency to produce electricity for captive use, or, if appropriate, also for export. Gas turbines are well known units to provide for electricity. Such machines generally consist of an air compressor, one or more combustion chambers in which gas or liquid fuel is burnt under pressure and a turbine in which the hot gases under pressure are expanded to atmospheric pressure. Since the high temperatures of the combustion gases produced would result in serious damage to the turbine blades if they were directed exposed thereto, the combustion gases are normally cooled to an acceptable temperature by mixing them with a large amount of excess air delivered by the compressor. About 65% of the total available power is consumed by the compressor, leaving 35% as useable power. A slight decrease in compressor efficiency reduces the amount of useful power, and, consequently, the overall efficiency considerable. By compressing the air in two stages with an intercooler in between increases the thermal efficiency of the gas turbine. So, the fuel availability is an important factor in optimising any gas turbine efficiency.
An additional constraint to be taken into account with respect to the use of gas turbines lies in the impracticability of using low-grade heavy fuels as feedstocks for gas turbines since turbine parts are easily corroded (even irrespective of the high temperature constraints described herein before) and fouled by sulphur compounds or ash (in particular vanadium compounds) and a very short life between overhauls can then be expected. Gaseous fuels or high-grade distillates seem to be the only practical fuels when continuous operation is necessary.
It is understandable that many efforts have already been devoted to the integration of various refinery operations in order to save costs. This has also been proposed for thermal conversion technology and electricity generation. Reference is made to the recent publication by F. A. M. Schrijvers, P. J. W. M. van den Bosch and B. A. Douwes in Proceedings NPRA, March 1999, San Antonio. In this publication, entitled “Thermal Conversion Technology in Modern Power Integrated Refinery Schemes” it is explained in detail how to integrate a so-called Thermal Gasoil unit with a gas turbine. One of the interesting aspects of such an integration is the use of a heat recovery unit downstream of the gas turbine which allows replacement of the conventional direct fired heater and soaker as well as the recycle heater for distillate.
Although this approach has important advantages compared with the use of conventional equipment, in particular because of the very low average and peak heat fluxes obtainable, it has no impact on the product slate of the thermal cracking operation in which still a large amount of residual material, usually referred to as vacuum flashed cracked residue (VFCR) is produced. Typically a Thermal Gasoil unit will produce between 45 and 65%, especially about 55%, by weight on feed of VFCR.
It would be desirable to use the residual material produced as feedstock for the gas turbine present in the integrated refinery operation. However, there are at least two major problems which prevent the direct use of VFCR as feedstock for the gas turbine. Firstly, VFCR type materials, like any heavy residue, are rich in unwanted sulphur compounds (which have, in essence, accumulated therein when compared with the initial feedstocks) which render them impracticable for duty as gas turbine feed as described herein above. Secondly, in an integrated operation only a very small fraction of the VFCR material produced would be needed (assuming that it did not have other constraints) to run the gas turbine, e.g. in the order of 2–5% by weight on feed which means that the vast majority of residual material would not be required for this duty thus causing a serious mismatch between the two operations to be integrated.